What is the difference between a psychologist and a psychiatrist and who is ideal to see first?

Difference in degree: A psychiatrist is a medical doctor (M.D.). A psychologist is a PH.D. Both can diagnosis and treat mental disorders. The difference is that M.D.'s can additionally prescribe medications and also have completed medical school like any other physician and are trained in multiple body diseases, treatments, and medications. Both usually work together and initially either should be fine.

Therapy vs meds: Speaking very generally, most psychologists hold either a PhD or a PsyD degree (I hold both) and practice psychotherapy. Psychiatrists hold either an MD or a DO degree, and they prescribe psychotropic medications, and generally do not do all that much therapy these days, but years ago psychiatrists would do both. If symptoms are severe, see the psychiatrist first.